The invention relates to a linear drive device wth two motors, in which one motor drives a nut, the other a screw spindle. Such a device in comparison with a device that works with only one motor, which drives the spindle, for example, has the advantage of a much greater range of control for the feed rate.
A creeping, very slow feed can be achieved if both motors have the same direction of rotation but are operated with a slight difference in rpm. A fast feed can be achieved if only one motor is operated and the other one remains inoperative an, finally, a very fast feed can be obtained if the motors are operated in opposite directions. Such linear drives are known, e.g., from PCT/CH No. 82/00129.
In these known linear drives a very slow feed is achieved by having both motors operate in the same direction of rotation, but with only a slight difference in the number of rpm. If the feed is to be extremely slow or in extremely small steps, step motors with electronic control are necessary that can make 100 or more steps per revolution. Such electronic controls, however, are very expensive.
The invention addresses the problem of creating a linear drive device of the desired kind which makes possible an extremely slow feed rate and extremely small feed steps more simply and more inexpensively.
The invention solves this problem with a linear drive device wherein one motor drives the screw spindle via first intermediate drive and the other motor drives the screw by means of a hollow shaft via a second intermediate drive. Additionally, the two intermediate drives have a very slight but precisely defined difference in their transmission ratio. This slight but precisely defined difference in the transmission ratios results with the same direction of rotation and the same number of rpm in a very low feed rate, which can be decreased even more by a change in the rpm. If step motors are used, feed steps under 1/1000 mm can be obtained.